System for and method of searching for targets in a marine environment

ABSTRACT

An above-the-water system for and method of finding targets, both animate and inanimate, in a marine environment, especially by determining the distance and depth of targets at, above or below the surface of, the water. An optical transmitter transmits infrared and ultraviolet light beams toward different zones of coverage on the water. An optical receiver equipped with a segmented detector separately detects return target reflections. An indicator, including range and depth indicators, provides information as to the distance to the target and, if it is below the water, its depth.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional application Ser.No. 60/167,995, filed Nov. 30, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to searching for targets in amarine environment and, more particularly, to warning of threats andperils in the marine environment, especially of impending collisions andterrorist threats.

2. Description of the Related Art

An optically-based, marine collision avoidance system was disclosed inmy earlier U.S. Pat. No. 4,290,043. A light beam was directed from atransmitter on-board a marine vessel at a surface of the water forrefraction into, and also for reflection from, the water. If no obstaclewas present in the path of the refracted beam, or in the path of thereflected beam, then the refracted and reflected beams continued theirpropagation away from the transmitter. If an obstacle was present, thenreturn light reflected off the obstacle by either the impingingrefracted beam or the impinging reflected beam was detected by areceiver on-board the vessel, and an alarm was sounded. The light beamincluded optical radiation having wavelengths in the ultraviolet range(300-400 nanometers) for increased water penetration and/or in theinfrared range (700-1500 nanometers) for increased water reflectivity.The light beam had wavelengths transmitted exclusively in one of theranges, or preferably had wavelengths simultaneously transmitted in bothranges.

As advantageous as my patented collision avoidance system was in warningvessel operators of impending collisions with an obstacle, it providedno information as to the precise location of the obstacle. Thus, mypatented system simply warned that an obstacle was present somewhere,but did not advise the vessel operator whether the obstacle wasunderwater, abovewater, or both. Moreover, my patented system did notadvise the vessel operator of the range of a floating or abovewaterobstacle, or of the depth of an underwater obstacle. This informationwould have been helpful in advising the operator how much warning timewas available to take evasive action, or if evasive action had to betaken altogether.

Other systems which I am aware include:

U.S. Pat. No. 5,042,942 which discloses the transmission of a laser beamfrom an overhead, tracking aircraft to an underwater body, especially atowed sled;

U.S. Pat. No. 5,146,287 which discloses an on-board laser scanner todetect floating or submerged hazards, such as mines or torpedoes;

U.S. Pat. No. 5,444,441 which discloses an arrangement for detectingunderwater objects by using a camera with three separate arrays ofdetectors to provide concurrent red, blue and green signals;

U.S. Pat. No. 5,646,907 which discloses detecting floating or submergedobjects by transmitting an amplitude-modulated laser beam to theobjects, and monitoring for acoustic echoes;

U.S. Pat. No. 3,644,043 which discloses a target search and track systemoperative, during a search mode, to detect infrared light from thetarget and, during a track mode, to detect laser light reflected fromthe target;

U.S. Pat. No. 4,047,816 which discloses the use of two lighttransceivers to track a vehicle in flight;

U.S. Pat. No. 5,345,304 which discloses a target acquisition system foruse on a flight vehicle and employs an infrared detector in combinationwith a laser sensor;

U.S. Invention Registration H341 which discloses a system for scanning alaser beam at high speed with high resolution, and also at low speedwith low resolution to steer a telescope at a target; and

U.S. Invention Registration H1231 which discloses a system for defendingagainst antiship torpedoes by initially employing sonar to determine theapproximate location of an incoming torpedo, and by also employing ablue-green laser to scan the located field of view.

SUMMARY OF THE INVENTION OBJECTS OF THE INVENTION

Accordingly, it is a general object of the present invention to improvethe state of the art as exemplified by U.S. Pat. No. 4,290,043 bydistinguishing among underwater targets, abovewater targets, and bothabovewater-and-underwater targets.

More particularly, it is an object of the present invention to providerange information as to a distance of an abovewater target relative to asystem transmitter.

Still another object of the present invention is to provide depthinformation as to a distance of an underwater target relative to asystem transmitter.

It is yet another object of the present invention to provide both depthand range information of a target, and even to estimate a size of thetarget.

A still further object of the present invention is to search for humantargets in search-and-rescue missions, and in terrorist threats.

Yet another object of the present invention is to search for inanimatetargets for collision avoidance.

An additional object of the present invention is to enable systemoperation twenty-four hours a day, and during different marineconditions.

FEATURES OF THE INVENTION

In keeping with the above objects and others which will become apparenthereafter, one feature of the present invention resides, briefly stated,in a system for, and a method of, searching for targets, both animateand/or inanimate, in a marine environment, comprising a transmittermeans, a processor including a receiver means, and an indicator.

The transmitter means is mounted abovewater, for example, on-board amarine vessel, an aircraft, or a seaside structure, such as a pier, abridge, a marina, or a boat dock. The transmitter means emits first andsecond beams of optical radiation at first and second zones of water.The first beam has a first wavelength characteristic having wavelengthsin the ultraviolet to blue range (300-475 nanometers), hereinaftersometimes referred to as a “blue” beam for simplicity, and capable ofentering the first zone of water and being refracted therethrough as arefracted beam. The second beam has a second wavelength characteristichaving wavelengths in the infrared range (650-1500 nanometers),hereinafter sometimes referred to as a “red” beam for simplicity, andcapable of reflecting from the second zone of water as a reflected beam.The blue and red beams are emitted either by separate transmitters, suchas laser light sources, or by a single light source, such as a xenonlamp having means for separating the wide spectrum output of the lampinto separate red and blue beams.

The processor is operative for identifying locations of the targets inthe marine environment. The receiver means is operative for separatelydetecting return target reflections reflected off any targets impingedby the refracted and/or the reflected beams to find an identifiedtarget. The return target reflection reflected off a target impinged bythe refracted beam, and the return target reflection reflected off atarget impinged by the reflected beam, are detected either by separatereceivers, or by a single receiver operating in a timewise alternatingmanner to detect a return target reflection from the refracted beamduring one time interval, and to detect a return target reflection fromthe reflected beam during a subsequent time interval.

The indicator is operative for indicating the identified target.Preferably, the indicator includes a display panel having separatedisplays for the red and blue beam reflections.

In contrast to the system of my earlier patent described above in whicha single beam was propagated forwardly of a vessel, the presentinvention directs at least two different beams towards at least twodifferent zones of water to obtain two different return targetreflections, one by the refracted beam, and another by the reflectedbeam, and separately detects the return target reflections. If the onlytarget reflection detected is from the refracted beam, then anunderwater target is identified. If the only target reflection detectedis from the reflected beam, then an abovewater target is identified. Iftarget reflections from both the refracted beam and the reflected beamare detected, then multiple targets are identified, or a single targetextending both above and below the water is identified.

In further accordance with this invention, the red and blue beams arepulsed, and the time width of each pulse and the spacing between pulsesare known. By determining the time duration from the moment atransmitted pulse is emitted until a corresponding received pulse isdetected, the distance or range to a floating or abovewater target canbe computed, as well as the depth to an underwater target. The indicatorcan also display the range and depth information.

Thus, this invention enables targets to be found and categorized withmore particularity. Human targets, such as a person requiring rescue,either in a small boat or not, can be identified as a floating target.Terrorists, such as a person or persons intent on causing damage forpolitical purposes, either swimming or being conveyed in a low- or high-profile boat, can be identified as a floating target prior to completingthe terrorist mission. Inanimate targets such as floating debris, orseaside structures such as a pier, a dock, a marina, and a bridge, orunderwater objects such as a sandbar, a reef or a mine, or abovewaterobjects such as a bridge support, or objects that extend both above andbelow the water such as an iceberg, are just a few examples of targetswhose depth, range, size and location can be determined by the presentinvention.

The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a system according to this inventionduring searching for targets in a marine environment;

FIG. 2 is a view analogous to FIG. 1 during detection of an underwaterobject;

FIG. 3 is a view analogous to FIG. 1 during detection of a floatingobject;

FIG. 4 is a view analogous to FIG. 1 during detection of an object thatextends both above and below the water and has a certain configuration;

FIG. 5 is a view analogous to FIG. 4 for an object having a differentconfiguration;

FIG. 6 is a general view of the system of FIG. 1;

FIG. 7 is a top plan view of the system of FIG. 6;

FIG. 8 is a perspective view of one embodiment of a transceiver;

FIG. 9 is a rear elevational view of a display panel of the transceiverof FIG. 8;

FIG. 10 is an enlarged sectional view taken on line 10—10 of FIG. 8;

FIG. 11 is a sectional view taken on line 11—11 of FIG. 10;

FIG. 12 is a schematic view of another embodiment of a transceiver;

FIG. 13 is a view of one embodiment of a receiver;

FIG. 14 is a front elevational view of the receiver of FIG. 13;

FIG. 15 is an overall view of a system according to this invention;

FIG. 16 is a view depicting geometrical relationships at work in thesystem;

FIG. 17 is a depiction of a transceiver at work in rough water;

FIG. 17a is a depiction of the detector during a test mode;

FIGS. 17b-e are successive depictions of the detector during red searchmode;

FIGS. 17f-i are successive depictions of the detector during blue searchmode;

FIG. 18 is a series of pulse diagrams depicting system operation;

FIG. 19 is a top plan, diagrammatic view of zones of coverage during thetest mode of FIG. 17a; and

FIG. 20 is a top plan, diagrammatic view of scanned zones of coverageduring the search modes of FIGS. 17b-17 i.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1-5 of the drawings, reference numeral 10generally identifies a transceiver mounted on a marine vessel 12 andemployed in a system for searching for targets such as an underwatertarget 14 in FIG. 2, a floating target 16 in FIG. 3, a part-underwaterand part-abovewater target 18 of a compact shape in FIG. 4, and apart-underwater and part-abovewater target 20 of a different, irregularshape in FIG. 5.

Although the vessel 12 is depicted as a powerboat, this was done tosimplify the drawings since any type or size of marine vessel can serveas a mount for the transceiver, including but not limited to motorboats,sailboats, rowboats, houseboats, yachts, catamarans, hydrofoils,warships, etc. The transceiver could equally well be mounted on anaircraft such as a helicopter or airplane operative for conductingsearch-and-rescue or reconnaissance missions in a marine environment.The transceiver can also be mounted on a seaside structure, such as apier, dock, marina, bridge, bridge support, etc. Whether mounted on astationary or a movable mount, the transceiver 10 is mounted on anabovewater support in a marine environment.

The targets need not be inanimate objects as shown, but could include aperson or persons requiring rescue, or terrorists threatening a marinevessel or seaside structure. Examples of inanimate targets includeunderwater or floating mines, reefs, sandbars, debris, icebergs, buoys,seaside structures, bridge supports, etc.

The transceiver 10 includes a transmitter circuit 22 (see FIG. 15) foremitting at least two beams of optical radiation. A first beam 24 has afirst wavelength characteristic in a wavelength range including blue andultraviolet light (300-475 nanometers) and is hereafter referred to as a“blue” beam for simplicity. A second beam 26 has a second wavelengthcharacteristic in a wavelength range including infrared light (650-1500nanometers) and is hereafter referred to as a “red” beam for simplicity.

The blue beam 24 preferentially penetrates seawater. The red beam 26preferentially reflects from seawater. The transmitter circuit 22 istilted down and faces the air-water interface or water surface 28 showngenerally horizontal in FIGS. 1-5 for simplicity. The blue and red beamsare directed along respective paths each extending away from thetransceiver and the vessel, and downwardly towards the water surface 28.

As shown in FIG. 1, blue beam 24 impinges on the water surface at firstzone A, penetrates the first zone, and is refracted in the water as arefracted beam 34. Red beam 26 impinges on the water surface at secondzone B, and is reflected from the water as a reflected beam 36. Asdescribed below, the zones A and B occupy different areas of the watersurface, with zone A preferably being closer to the transmitter circuitthan zone B.

The drawings of FIGS. 1-5 are simplistic renderings because, as shown inFIGS. 6 and 7, each of the blue and red beams are generallyconically-shaped. As considered in a vertical plane (FIG. 6), the bluebeam 24 is bounded by an upper ray 24 a, a lower ray 24 c, and a centralray 24 b which lies along the optical axis of symmetry. As considered ina horizontal plane (FIG. 7), the blue beam 24 is bounded by a right ray24 d and a left ray 24 e. Similarly, the red beam 24 is bounded by anupper ray 26 a, a lower ray 26 c, a central ray 26 b which lies alongthe axis of symmetry, a right ray 26 d and a left ray 26 e. The zones Aand B are marked in FIG. 7 and, as shown, are offset although, in somecases, they may partially overlap.

Returning to FIGS. 1-5, the beams 24, 26, 34, 36 are not shown asconically-shaped, but instead, are represented by their respectivecentral rays in order not to unduly encumber the drawings. Thetransceiver 10 is adjustably mounted on a mount 30 so that the zonesimpinged by the blue and red beams 24, 26 are controlled. Any acuteangle can be selected for the angle of inclination that a respectivebeam makes with the horizon. The inclination angle depends also upon thelocation of the transceiver relative to the mount 30 and the watersurface. As described below, optical elements in the path of therespective beams control the dimensions of zones A and B and theirlocation relative to the transceiver. Preferably, the inclination angleis on the order of 0.1° to about 3°.

The transceiver 10 includes a receiver circuit 32 (see FIG. 15)controlled by a control processor 42 and operative for separatelydetecting return target reflections reflected or scattered off anytargets impinged by the refracted beam 34 and/or the reflected beam 36in order to find a target and identify its location in the marineenvironment. In FIG. 1, the refracted and reflected beams 34, 36continue their propagation away from the transceiver because there is noobstacle in their paths to scatter impinging beams. In FIG. 2, thesubmerged obstacle 14 scatters only the refracted beam 34 and redirectsreturn target reflections 44 to the receiver circuit. In FIG. 3, thefloating obstacle 16 scatters only the reflected beam 36 and redirectsreturn target reflections 46 to the receiver circuit. In FIG. 4, theobstacle 18 scatters both the refracted and the reflected beams 34, 36and redirects return target reflections 44, 46 to the receiver circuit.In FIG. 5, the obstacle 20 also scatters both the refracted and thereflected beams 34, 36 and redirects return target reflections 44, 46 tothe receiver circuit. As described below, range and depth informationfor the individual return target reflections help to distinguish thedifferent configurations of the obstacles 18, 20. This information isvisibly displayed on a display panel 40 (FIG. 9).

As also described below, an annunciator circuit 38 (see FIG. 15) isoperative for indicating the presence of the identified target. Theannunciator circuit 38 can emit one warning for each target, ordifferent warnings for different targets. The warnings can be audibleand/or visible.

FIGS. 8-11 illustrate a preferred embodiment of the transceiver 10. Thetransceiver includes a housing 50 having a hood 52 for shielding a frontface 54 from direct sunlight. The display panel 40 may be situated at arear face of the housing 50, but preferably is mounted away from thetransceiver within sight of an operator. A pair of stub shafts 56supports the housing for pivoting movement on a fork 58. The housing 50is mounted on an upper end of a stabilizer rod 60 whose lower end isconnected to a stabilizer weight 62. The mount 30 includes a deckmounting plate 64 and a hollow cylindrical tube 66 on which the fork 58is mounted. The rod 60 and weight 62 are free to move within the tube 66as the angular attitude of the vessel changes relative to the watersurface to maintain a vertical orientation.

The transmitter circuit 22 and receiver circuit 32 are mounted in awatertight casing 64 within the housing 50. Entry and exit vents in thehousing, assisted if necessary by a cooling fan (not illustrated),provide cooling of the transmitter/receiver circuits. An electricalcable 68 supplies electrical power to these circuits.

The transmitter circuit 22 includes a blue laser 74 for emitting theblue beam 24, and a discrete red laser 76 for emitting the red beam 26.The lasers may be tubes or diodes. Springs 70 urge the lasers towardexit windows 72. Drivers 78 energize the lasers 74, 76. The windows 72can incorporate focusing lenses, or separate focusing lenses can be usedto optically modify the beams 24, 26.

The receiver circuit 32 includes a detector 80 having an arrayed surfacedivided, as shown in FIG. 14, into multiple rows 1-8 and columns A-I ofcells or pixels. A blue filter 84 covers the upper half of the detectoror rows 1-4. A red filter 86 covers the lower half of the detector orrows 5-8. The filters 84, 86 can be discrete or integral. A focusinglens 88 is located in front of the filters 84, 86 and directs thereturning blue rays 44 to the upper rows 1-4 through the blue filter 84,and also directs the returning red rays 46 to the lower rows 5-8 throughthe red filter 86. The focusing lens 88 can serve as an entrance windowon the housing 50, or a separate window can be provided.

As previously noted, the red beam 26 reflects off the water. Actually, asmall amount of the red beam penetrates the water. However, this smallamount is negligible. More importantly, the blue beam 24 refracts intothe water; however, a small, non-negligible amount of the blue beamreflects off the water. If this blue beam reflection strikes an obstacleat or above the surface, its reflection towards the receiver circuitmakes it impossible to determine whether or not this reflection comesfrom an object above or below the surface. A requirement of this systemis that the red beam is used only for objects at or above, and the bluebeam is used only for objects at or below, the water's surface.

Hence, the filters 84, 86 specifically direct one beam to impinge on onehalf of the area or field of view of the detector, and the other beamfalls on the other half or the remaining field of view. In other words,the filters 84, 86 restrict the red and blue reflections to specificareas of the detector. Now, due to the tilt of the transceiver relativeto the water, blue reflections can only be associated with obstaclesthat are at and below the surface, and red reflections can only beassociated with targets at and above the surface.

An ambient light detector 90 is mounted on the housing pointing upwardsaway from the water surface. The detector 90 is covered by a translucentdome that averages the ambient light. This detector 90 is used toautomatically adjust the gain of the main detector 80. The transmitters74, 76 and the detector 80 are mounted in separate housings such thatnone of the emitted beams can go directly to the detector 80.

In accordance with another embodiment depicted in FIG. 12, a xenon flashtube 92 is a rich source of both red and blue light. Therefore, only onetransmitter is needed in this transmitter circuit. However, whereas thered and blue wavelengths of the dual lasers discussed above are selectedto be invisible to the eye, the xenon lamp requires a filter 94 toaccomplish this task.

The xenon lamp is plugged into a trigger socket 96. Filter 94 and afocusing lens 98 are located in front of the lamp 36. The detector 80has a red/blue split filter 84, 86 and the lens 88 in front of it. Anaperture stop 100 in front of the detector 80 limits the field of viewof the detector 80. The ambient light detector 90 faces upwards and hasa translucent dome over it in order to average ambient light.

An optical prism 102 separates the red and blue light into two separatebeams. In this way, even a single emitter source can be used so thateach of the main wavelengths, red and blue, may be used with optimumefficiency. Note, that without the wedge, all of the emitted red lightand all of the emitted blue light would be superimposed on the same areaon the water. The blue light at the farthest edge of the illuminatedpattern on the water would not be able to refract into the water. Thered light at the nearest edge of the pattern (close to the transceiver)would not be able to refract into the water. There would be asignificant loss of energy, and system performance would bedeteriorated.

Hence, the transmitter circuit can have one or more transmitters.Likewise, the receiver circuit can have one or more detectors.

The control processor 42 in the preferred embodiment controls thedrivers 78 and pulses the blue and red beams with a known time width foreach pulse and a known spacing between pulses. Preferably, the blue andred beams are pulsed alternatingly. The processor 42 includes a timer104 for counting the time duration from the moment each pulse istransmitted until it returns as a reflection to the receiver in order tocompute the distance to the target.

Returning to the display panel 40 of FIG. 9, the detection of a targetreflection 44 is indicated by a “blue” light indicator 110, and thedetection of a target reflection 46 is indicated by a “red” lightindicator 112. The computed distance between the moment a blue beampulse is emitted and the detection of a received blue beam reflection isindicated numerically by a blue range indicator 114. The computeddistance between the moment a red beam pulse is emitted and thedetection of a received red beam reflection is indicated by a red rangeindicator 116. The computed depth between the water surface and theheight of a submerged object is indicated by a blue depth indicator 118.The depth computation takes into account the different speed of bluelight in air and in seawater and, of course, the index of refraction ofseawater. An on/off switch 120 is also located on the display panel.

FIG. 16 illustrates the geometry in more detail. There are as many“specific rays” in a system as there are pixel rows. FIG. 13schematically shows eight pixel rows so that there are eight specificrays. If the detector has 256 rows, then there are 256 specific rays.Each ray is identified by the pixel row number on which it impinges.

Knowing the height H (FIG. 16) of the transceiver above the watersurface 28 and the inclination angle (angle Φ) of the specific rayrelative to the horizon, the length of the specific ray (from the lens88 to the water) may be calculated. Although every specific ray has thesame height, each has a different inclination angle and a differentlength. The arithmetic value of this length is stored in a memory bankof a microprocessor in the processor 42.

The pixel or cell is the smallest element of resolution. It is notpossible to identify where the light is falling on the individual pixel.The pixel's location, from row 1 to row 8, defines “specific rays” 1 to8. For example, specific ray 2 impinges on pixel row 2 and specific ray4 impinges on pixel row 4. In the view shown in FIG. 13, each row ofpixels identifies a specific ray which, based on the focal length of thelens 88, defines a specific angle. The angle formed by the size of thepixel and the focal length of the lens 88 defines the limitingresolution of the system.

FIG. 16 illustrates the angle of inclination Φ and a representativespecific ray SR#_(air). The angle α# must be added to inclination angleΦ to form the specific depression angle Da#. This angle is (Φ+α#). Theequations that determine SR#_(air) are as follows:

α′#=arctan x#/f  1.

α=α′  2.

Da=(Φ+α#)  3.

sin (Φ+α#)=H/SR#  4.

SR# _(air) =H/sin (Φ+α#)  5.

calculated time_(air) =SR# _(air)/pulse width  6.

SR# _(water)=[(measured time/2)−(calculated time)]×pulse width/n′_(water)  7.

Depth =SR# _(water)×cos I′  8.

distance_(air) =SR# _(air)×cos (Φ+α#)  9.

distance_(water) =SR# _(water)×sin I′  10.

total distance=distance_(air)+distance_(water)  11.

warning time=total distance/velocity  12.

where α is the offset distance of each row relative to the optical axis,f is the focal distance of the lens 88, α′# is the angle subtended byeach row between the lens 88 and the detector 80, α# is the anglesubtended by each ray between the lens 88 and the target, SR#_(air) isthe distance in air of each specific ray between the lens and the watersurface, SR#_(water) is the distance in water of each specific raybetween the water surface and the target, n′ is the index of refractionof seawater, and I′ is the angle subtended by the refracted ray.

Of critical importance is the equation for the path length of thespecific ray SR#_(water). Basically, the underwater path SR#_(water) isdetermined by subtracting the total measured time of flight of thepulse, minus the calculated time that the pulse travels in air. Theremainder is the time that the pulse spent underwater. This time is thencorrected by the index of refraction of the water. See equation 7.

As previously stated, the system's geometry is dependent on the heightof the transceiver above the water. Obviously, during rough seas, thebow of a vessel will “bob” up and down. The transceiver is gimbaled, andwill maintain its inclination angle Φ, but certainly the height will beconstantly changing. Another feature of this invention provides meansfor constantly monitoring and measuring the height of the transceiverabove the water. It also provides means for manually entering the heightand inclination angle into the processor 42 during initial systeminstallation.

Actually, it is not height that is important. It is the air distance,from the transceiver to the water surface 28, of each specific beam thatmust be determined. To do this, consider that there are two sea statesof interest, calm water and rough water. Calm water is defined as nothaving whitecaps. Rough water is defined as having whitecaps 122 (FIG.17). In calm water, red beam 26 reflects away from the transceiver. Theprocessor expects a return signal from a reflection at a specific time.If it does not receive this signal, the processor makes thedetermination that the water is calm, and the height H of thetransceiver is known.

On the other hand, if the water is rough, the red beam 26 will reflectfrom a whitecap back towards the transceiver, as shown in FIG. 17. Uponreceiving this signal, the processor compares the time that the specificray returns with the time stored in the memory bank for that specificray. If the time is different, the processor computes a new air distanceand does this for each and every specific ray in the system and storesthese new values in memory. This process occurs several hundred times asecond. Each interrogation of the detector “freezes” the up and downmotion of the ship. A special detector interrogation mode is used solelyto determine the height (or specific ray's length) of the transceiverabove the water.

A special pulse mode is used to determine the air distance of a specificray. During a first test mode, the gain of the detector 80 is adjustedto a maximum value based on an ambient signal input from the ambientlight detector 90. A red beam pulse is emitted from the red transmitter76 and, at the appropriate time, a few red pixels in rows 5-8 areinterrogated, as schematically shown in FIG. 17a. If the water is calm,no return signal is detected. No signal indicates calm water and thestored specific ray air distances remain, without change, in memory.This is repeated several times a second. The detector pixels that areinterrogated form a “matrix pattern”, which as shown in FIG. 17aincludes five pixels. This pattern, as well as the ambient lightdetector's signal, are the foundation for monitoring the air distanceand, hence, the height of the vessel. Of course, if a signal is receivedduring the test mode, the processor immediately calculates and adjustsall the stored air distances.

The next mode is called the “red search mode”. During this second mode,the gain of the detector 80 is adjusted so as to not detect whitecaps.Now, the detector is interrogated, row by row (or pixel by pixel). Theinterrogation sequence occurs as shown in FIGS. 17b-17 e. If a signal isreturned, the information is processed and the appropriate alarm andrange information are displayed.

The third and final mode is called the “blue search mode”. It operatesin the same manner as the red search mode except that a differentsection of the detector is interrogated as shown in FIGS. 17f-i. If asignal is detected, the depth and range will be displayed.

As depicted in the pulse sequence diagrams of FIG. 18, a red laser pulse130 is emitted and followed by a time delay 132. The detector 80 isplaced into “test mode” and, if the water is rough, a signal 134 isreceived. The amplitude of the signal 134 is shown double height toindicate that the ambient light signal detector 90 has adjusted the gainof the main detector 80 to the maximum level. The time it takes for thesignal 134 to arrive is used by the processor to determine the length ofthe specific rays.

The system now shifts to “red search mode” and the detector gain islowered to a value that will not detect whitecaps. A second laser pulse138 is emitted. There is a time delay until pulse 140 returns. Thispulse 140 is shown in normal amplitude. If there is no obstacle, thenthis signal 140 would not be evident. If a signal 140 is present, thenthe alarm is sounded, and the range to the obstacle is displayed.

FIG. 18 shows pixel rows 1 through 8 as having received signals. Thismeans that these diagrams represent an obstacle that is located aboveand below the water and is equi-distant from the vessel. All thereturned signals have the same time delay.

This pulsing sequence is repeated several to hundreds of times a second.The frequency of repetition is dependent on specific systemrequirements. The pulsing sequence of this invention is not limited tothat shown in FIG. 18. In some embodiments, the output shape of thelight pattern may be different such as a line scan. Obviously, theinterrogation of the detector would follow that specific pattern.

The red test mode is a special feature of this invention. The shadedareas 83 shown in FIG. 19 represent areas on the water that would beimaged on the shaded pixels 81 of the detector 80 shown in FIG. 17a.These pixels may be interrogated simultaneously or sequentially. Thesole purpose of the test mode is to detect the presence of whitecaps andwhitecaps only.

During the test mode, a reflection from either a target or whitecapsdoes not and cannot sound an alarm. Instead, the signal is processed todetermine specific ray lengths and is used in conjunction with theambient light signal to set the gain of the main detector 80 prior tothe emission of a search mode pulse. Of course, it is possible that anobstacle may be located somewhere in the red zone during the test mode.The matrix pattern is designed to minimize any chance that a target willbe located in test area 83 such as shown in FIG. 19. Even if it islocated in an area seen by a pixel, its reflection will be averaged withthe signals received by all five pixels. That means that the reflectancefrom the target will represent only 20% of all the energy seen by thedetector 80 during the test mode.

Although only five pixels are depicted for purposes of discussion, somedetectors will have many more pixels. The smaller each pixel, thesmaller the projected test area on the water and the less chance that anobstacle will influence signals during the test mode. Even if theobstacle is extremely large, it is unlikely to be large enough tointercept any more than one of the five pixel water patterns.

The amplitude of the test mode signals received by the detector 80 isadded to the ambient light signal received from ambient light detector90. This combined signal represents total background illumination ofwater and sky. This illumination decreases the dynamic detection rangeof the detector 80. It is noise, and it can cause false alarms. The gainof the detector is automatically changed to diminish the effects of thisbackground illumination. This gain adjustment occurs after each andevery test mode and just before a search mode is initiated.

The ambient light detector's 90 translucent dome is designed and locatedso that it responds to all background illumination that will influencethe detector 80, with the exception that it does not cover the area ofthe water that the detector 80 does. In this manner it is simple toisolate the signals coming from whitecaps and that coming from all othersources. For example, at night time and during calm water, there will beno significant ambient light or any reflection from whitecaps. Underthese conditions the detector gain will be set to a maximum. Duringrough seas at night time, only the whitecaps will affect the gain.During calm waters in the day time, the background illumination seen bythe ambient light detector 90 will adjust detector 80 gain. If, duringthe same ambient illumination, the water becomes rough, then only theaverage signal coming from the detected whitecaps will adjust the gain.The signal from the ambient detector 90 will remain the same. Then, onlya target of a size and reflectivity that produces a signal somewhatlarger than that produced by the whitecaps during the test mode, willproduce an alarm.

Small changes in the inclination angle results in large changes in thecomputed depth of a submerged target, as well as the warning time. Inone geometry, a difference of 2° in inclination angle results in afive-fold difference in detection depth, and about a two-fold increasein the warning time.

As the wavelength of the blue and red beams 24, 26 approach the size ofair molecules, atmospheric backscatter can be diminished by physicallyseparating the transmitter and receiver circuits and/or by gating thereceiver circuit, especially if the emitted beam pulses are narrow inwidth. A 10 ns pulse width covers about 10 feet in air. If the receivercircuit is turned off for the first 100 feet of travel by an emittedbeam pulse, and thereupon turned on for a period of time during which atarget reflection is expected to be returned if a target is present,then the backscatter is significantly reduced and, in effect, enablesthe system to effectively operate in “fog” conditions.

Gating also minimizes the triggering of false warnings caused by roughseas. Even on windless days, the surface of the water exhibits asubstantial degree of optical irregularity. On windy days, ripples,whitecaps, and even the smooth water portion of whitecapped wavesdisperses the incident beams in a random manner. Some portion of therefracted and reflected light is totally lost, and some of thisdispersed energy may reach the receiver circuit and trigger a falsealarm.

A single pulse of a blue beam is emitted from the transceiver. From thetime the pulse is emitted until the time the blue beam enters the water,a shutter or gate in the receiver circuit is kept closed. Not until theblue beam gets below the ripples (or whitecapped water) underneath thewater surface is the receiver gate opened. Even though a portion of theemitted pulse reflects from the water surface back towards the receivercircuit, it cannot be received or processed as a false alarm signal.Only the blue reflection reflected from, and only from, an underwatertarget, can sound an alarm.

Hence, it is desired that the gate be opened so that a target at, orbelow, a “gate line” underneath the water surface may be detected. It isalso important that the gate line be parallel to the water's surface.Specific rays of the blue beam have a different length, or to put itanother way, take different times to reach the water. To make the gateline parallel to the water, the gate for each specific ray must open andclose at a different time. Since each specific ray reports to a specificpixel row, the interrogation of each pixel row is done insynchronization with the emitted pulses.

Therefore, the gate is opened and closed, pixel row by pixel row, basedon the pixel row that is being interrogated. The processor changes thegate time for each specific blue ray and its associated pixel row. Theinformation for adjusting the time of each pixel row gate is obtainedfrom the test mode. The test mode is constantly updating the processorwith geometric information on the length of the specific rays.Obviously, the length of the specific rays is a means of knowing exactlythe location of the water surface in relationship to the transceiver.

Although a large area of the water is being covered by the transmittercircuit, only a line scan portion, equivalent to a projected image of apixel row on the water, is being seen by the receiver circuit.

The efficiency of the radiant output power of the transmitter circuitmay be significantly increased (laser, xenon, mercury arc or otherradiant sources) if the projected beam energy is shaped into either alinear area or a spot. As shown in FIG. 20, four scan areas 150 areshown for the red zone B, and four scan lines 152 are shown for the bluezone A. Scan area 8 of the red beam on the water corresponds to matchingpixel row 8 of the detector 80 (FIG. 17b). After pixel row 8 isinterrogated, the red beam is moved to position 7 and, once again, pixelrow 7 is interrogated (FIG. 17c). In this manner, row by row of thedetector is gated and scanned, while looking for a returned targetreflection.

The same approach may be used with a projected blue or red beam shapedinto a spot. In this instance, each individual pixel will beinterrogated one by one.

Regardless whether a line or spot scan method is used, a means formoving the beam in a prescribed pattern must be included in thetransmitter circuit. Acousto-optic crystals, scanning mirrors, vibratinglens, and other mechanical means are useful ways of steering theprojected light beams.

As described so far, a single pulse of light striking a target isdetected by a receiver. An alarm is sounded, if and when, the returnedtarget reflection is above a preset threshold. This threshold is set bythe ambient light detector 90 controlling the gain of the detector 80.However, if a target's location, size, or reflectivity returns areflection whose signal strength is not above the single pulse presetthreshold, then the received signal is stored in an integrator circuit.The integrator circuit accumulates the received signals in the form of astored voltage. The amplitude of each stored voltage is added and, whenthe value of the amplitude exceeds the threshold, then the alarm issounded. The integrator significantly improves warning time and isanother advantageous feature of this invention.

Hence, this invention is not only generally useful for locating targets,both animate or inanimate, but also is useful as a marine obstaclecollision avoidance system mounted on a marine vessel, that not onlyalerts a helmsman as to the potential danger of a collision with amaritime obstacle, but also gives warning as to its location. The systemreveals whether the obstacle is above, at, or below the surface of thewater and gives the distance and depth of the obstacle. The systemoperates during the day and the night and in fog.

The system includes a transmitter circuit mounted on the vessel abovethe air-water interface. The transmitter circuit emits two beams, one inthe infrared (red) region, and the other in the ultraviolet (blue)region of the optical part of the electromagnetic spectrum. The red andblue sources of radiation may be generated by a single source, or byseparate sources.

The system includes a processor for pulsing, in an alternating manner,the discrete red and blue light beams. This entirely eliminatescrosstalk and signal processing errors between the red and blue beams.During the firing of a red beam pulse, if a return reflection isdetected, it can only be from an object at, or above, the water. The redbeam's wavelength cannot penetrate into the water. If a blue returnreflection is detected, then it can only be from an object at, or below,the water. If both signals are detected, then it is from an object thatis both below and above the water.

The red and blue beams are directed to, and projected on, distinct andseparate zones on the water. The red zone is optimized for detectingobjects at or above the water while the blue zone is optimized to detectobstacles at or below the water.

The system includes a receiver circuit equipped with an arrayed detectorand a special split red/blue filter that eliminates unwanted, above thesurface, blue reflections. This filter also serves the purpose ofminimizing unwanted background illumination, and making it possible todetermine whether an obstacle is below, at, or above the waterline.

If the transmitter circuit consists of two separate lasers, then the redlaser is aligned to point further away from the vessel than the bluelaser. Such misalignment constructs a separate red zone and a separateblue zone. This distinctly novel arrangement significantly improveswater depth penetration and warning time.

If the transmitter consists of a single radiation emitting source suchas a xenon flash tube, then it is equipped with an optical wedge orprism. This wedge directs the emitted red and blue beams along differentdirections and projects onto separate red and blue zones on the water.Although this approach does not allow for alternate pulsing of the redand blue beams, it, nevertheless, represents a significant improvementover the art.

According to another feature of this invention, the transmitter/receiverunit is maintained at a constant line of sight regardless of theattitude of the vessel. Also, the transmitter/receiver unit protects allinterior components from the harsh marine environment and providesproper cooling by using waterproof vents. The unit itself is removablefrom its mount for security reasons.

In a red test mode of operation, the geometric height (or length of farfield specific rays) of the system above the water's surface isdetermined.

A range gating feature not only penetrates fog, but also eliminatesunwanted reflections from whitecaps.

A linear or spot scanning transmitter synchronized with an interrogationof pixel rows or single pixels respectively of a detector optimizes thepower density output of a radiant source. This improves the system'sability to detect obstacles at longer ranges and greater depths.

An integrator is employed to improve system detection of very small, orvery distant, targets. The integrator stores signals that are below analarm threshold. If there really is an obstacle, then the integratoradds each received signal until the sum of the signals is above thethreshold, and then sounds the alarm.

Hence, the projection onto separate zones, and the separate detection ofreflections, of red and blue emissions is employed to detect not onlythe location, but also the size, of obstacles to be avoided in a marineenvironment. Distance of the obstacle is determined by pulse counting.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied in asystem for and method of searching for targets in a marine environment,it is not intended to be limited to the details shown, since variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention.

For example, more than one transceiver can be mounted on a vessel. Also,a single transceiver can be mounted for rotation or for oscillationabout a vertical axis and serve as a search beacon.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

I claim:
 1. A system for searching for targets in a marine environment, comprising: a) an abovewater-mounted transmitter means for emitting and directing, at a first zone of water, a first beam of optical radiation having a first wavelength characteristic enabling the first beam to refract through the first zone as a refracted beam, and for emitting and directing, at a second zone of water different in location than, and spaced apart from, the first zone, a second beam of optical radiation having a second wavelength characteristic different from the first wavelength characteristic and enabling the second beam to reflect from the second zone as a reflected beam; b) a processor for identifying locations of the targets in the marine environment, including a receiver means for separately detecting return target reflections reflected off any targets impinged by the refracted and the reflected beams to find an identified target; and c) an indicator responsive to the processor, for indicating the identified target.
 2. The system of claim 1, wherein the first wavelength characteristic includes a range of wavelengths having 475 nanometers as an upper limit, and wherein the second wavelength characteristic includes a range of wavelengths having 650 nanometers as a lower limit.
 3. The system of claim 1, wherein the first and second beams are angularly spread apart by an acute angle.
 4. The system of claim 1, wherein the first zone is closer to the transmitter means than the second zone.
 5. The system of claim 1, wherein the transmitter means includes a first transmitter for transmitting the first beam, and a second transmitter for transmitting the second beam.
 6. The system of claim 1, wherein the transmitter means includes a single transmitter for transmitting a common beam, and an optical modifying component for separating the common beam into the first and the second beams.
 7. The system of claim 1, wherein the indicator indicates that the identified target is an underwater target when the return target reflection detected by the receiver means is solely from the refracted beam.
 8. The system of claim 1, wherein the indicator indicates that the identified target is at least partly an abovewater target when the return target reflection detected by the receiver means is solely from the reflected beam.
 9. The system of claim 1, wherein the indicator indicates that the identified target extends at least partly abovewater and partly underwater when the return target reflections detected by the receiver means are from both the refracted and the reflected beams.
 10. The system of claim 1, wherein the receiver means includes a first optical filter for allowing only the return target reflection from the refracted beam to reach the receiver means, and a second optical filter for allowing only the return target reflection from the reflected beam to reach the receiver means.
 11. The system of claim 10, wherein the receiver means has an array of light detectors, and wherein the first optical filter covers a part of the array, and wherein the second optical filter covers a remaining part of the array.
 12. The system of claim 1, wherein the processor controls the transmitter means for alternatingly emitting the first and the second beams at different times, and wherein the receiver means is operative for alternatingly receiving the return target reflections from the refracted and the reflected beams.
 13. The system of claim 1, wherein the processor includes a timer for timing a first time duration between the emitting of the first beam and the detecting of the return target reflection of the refracted beam, and for timing a second time duration between the emitting of the second beam and the detecting of the return target reflection of the reflected beam; and wherein the processor includes means for determining a depth from the first time duration, and for determining a range from the second time duration.
 14. The system of claim 12, wherein the processor controls the transmitter means to emit each of the first and the second beams as a series of pulses.
 15. The system of claim 1, wherein the processor includes means for measuring a height above the water of the transmitter means relative to the water surface.
 16. The system of claim 1, wherein the processor includes means for sweeping the first beam over the first zone, and for sweeping the second beam over the second zone.
 17. The system of claim 1, wherein the processor controls the receiver means to initially detect return target reflections from the first zone during a first search mode, and to subsequently detect return target reflections from the second zone during a second search mode.
 18. The system of claim 1, wherein the processor includes means for calibrating the receiver means during a test mode by detecting reflections from the water at a plurality of spaced-apart areas within the second zone.
 19. The system of claim 1, wherein the processor activates the receiver means to be operative only after the first beam is below the water.
 20. The system of claim 1, wherein the processor controls the receiver means to be operative only after a respective beam has been emitted.
 21. A method of searching for targets in a marine environment, comprising the steps of: a) emitting and directing, at a first zone of water, a first beam of optical radiation having a first wavelength characteristic enabling the first beam to refract through the first zone as a refracted beam, and emitting and directing, at a second zone of water different in location than, and spaced apart from, the first zone, a second beam of optical radiation having a second wavelength characteristic different from the first wavelength characteristic and enabling the second beam to reflect from the second zone as a reflected beam; b) identifying locations of the targets in the marine environment, by separately detecting return target reflections reflected off any targets impinged by the refracted and the reflected beams to find an identified target; and c) indicating the identified target.
 22. The method of claim 21, wherein the first wavelength characteristic includes a range of wavelengths having 475 nanometers as an upper limit, and wherein the second wavelength characteristic includes a range of wavelengths having 650 nanometers as a lower limit.
 23. The method of claim 21, wherein the first and second beams are angularly spread apart by an acute angle.
 24. The method of claim 21, wherein the identified target is an underwater target when the return target reflection detected is solely from the refracted beam.
 25. The method of claim 21, wherein the identified target is at least partly an abovewater target when the return target reflection is solely from the reflected beam.
 26. The method of claim 21, wherein the identified target extends at least partly abovewater and partly underwater when the return target reflections detected are from both the refracted and the reflected beams.
 27. The method of claim 21, wherein the first and the second beams are alternatingly emitted at different times, and wherein the return target reflections are alternatingly received from the refracted and the reflected beams.
 28. The method of claim 21, and further comprising the step of timing a first time duration between the emitting of the first beam and the detecting of the return target reflection of the refracted beam, and timing a second time duration between the emitting of the second beam and the detecting of the return target reflection of the reflected beam; and the step of determining a depth from the first time duration, and determining a range from the second time duration.
 29. The method of claim 27, wherein the first and the second beams are emitted as a series of pulses.
 30. The method of claim 21, and further comprising the step of measuring a height above the water.
 31. The method of claim 21, and further comprising the step of sweeping the first beam over the first zone, and sweeping the second beam over the second zone.
 32. The method of claim 21, wherein the return target reflections from the first zone are initially detected during a first search mode, and the return target reflections from the second zone are subsequently detected during a second search mode.
 33. The method of claim 21, and further comprising the step of determining a condition of the water by detecting reflections from the water at a plurality of spaced-apart areas within the second zone.
 34. The method of claim 21, wherein the detecting step is performed only after the first beam is below the water.
 35. The method of claim 21, wherein the detecting step is performed only after a respective beam has been emitted. 